1. Field of the Invention
The present invention relates to an electrolyte solution capable of providing a lithium secondary battery having excellent cycle characteristics and excellent battery characteristics such as electrical capacity, storage characteristics etc., and an electrolyte thereafter.
2. Description of the Related Art
Lithium secondary batteries are widely used as a driving electric source for small-sized electronic equipment. Lithium secondary batteries are mainly composed of a cathode, a non-aqueous electrolyte solution and an anode. They can use a lithium composite oxide such as LiCoO.sub.2 as a cathode and a carbon material or lithium metal as an anode. As an electrolyte for a lithium secondary battery, carbonates such as ethylene carbonate (EC), propylene carbonate (PC) are preferably used.
However, secondary batteries having further improved characteristics such as cycle characteristics and electrical capacity are required.
In the case of a lithium secondary battery using, as an anode, a highly crystallized carbon material such as natural graphite or artificial graphite, peeling off of the carbon material is observed which causes problems, depending upon the degree of the phenomenon thereof, the battery capacity is decreased. The peeling-off of the carbon material is considered to occur due to the decomposition of the solvent in the electrolyte solution during the charge, which is considered to be due to the electrochemical reduction of the solvent at the interface between the carbon material and the electrolyte solution. Among the cyclic carbonate, propylene carbonate (PC) having a low melting point and a high dielectric constant is preferable, as the non-aqueous solvent, due to the high electric conductivity at a low temperature. However, when graphite is used as an anode, the decomposition of PC becomes remarkable, and therefore, PC was not able to be used as an electrolyte for a lithium secondary battery. Further, when ethylene carbonate (EC) is used, a part of EC is decomposed during the repeated charge and discharge cycle to cause decrease in the battery characteristics. Thus, it is the present status that the battery characteristics of the lithium secondary batteries such as the cycle characteristics and the electric capacity are not necessarily satisfied.